Multimedia application programs for workstations or computers are becoming more widespread. An example of a multimedia application program is the provision of full motion video to a workstation display and synchronized audio to workstation speakers.
A tremendous amount of storage is required for full motion video. A common technique for reducing the amount of storage requirements for video files is to compress the video data. The video data is then decompressed before being displayed. But even with data compression techniques, the amount of storage required for full motion video is very large. For example, using a particular type of compression and decompression technology referred to in the industry as digital video interactive, a typical data file contains about 9 megabytes of data for every 60 seconds of playback time.
One way to avoid having full motion video data files take up too much storage space on an individual workstation is to utilize a digital video server. Referring to FIG. 1, the digital video server 11 services plural workstations 13 that are connected to a token-ring local area network 15. The server 11, which is a high performance file server, accesses a data storage device 17, such as a hard disk drive. The compressed data files are stored on the data storage device 17 and transferred over the network in the compressed format. Each workstation 13 on the local area network (LAN) 15 has a display and speakers. In addition, each workstation 13 has a display adapter 19 that decompresses the video data for display on the workstation display. One type of display adapter is known as the Action Media II display adapter, available from IBM (IBM is a registered trademark of International Business Machines Corporation).
To access stored video data files, a workstation 13 sends a request along the network to the digital video server 11. The digital video server 11 retrieves the digital data file from the storage device 17 and sends it digitally on the LAN 15 to the requesting workstation.
The digital video server system described above suffers from several disadvantages. First, the LAN 15 presents a bottleneck to the distribution of video data to a large number of workstations. For example, in order to sustain full motion video at each workstation, the file server and token-ring network must deliver data to each workstation at a sustained rate of about 1.2 megabits per second (utilizing digital video interaction compression technology). We have determined experimentally that it is possible to sustain 10 simultaneous full motion video applications running on a 16 megabit per second token-ring network, while still leaving some bandwidth on the network to support other network traffic. However, a network may have more than 10 workstations that wish to simultaneously use full motion video data files.
There is currently no satisfactory method to ensure that video data can be delivered to each workstation at a constant, required (or guaranteed) rate. Successful operation depends on either limiting the number of video data enabled workstations on the network to a serviceable amount or limiting the number of simultaneous video applications by an informal agreement among workstation users.
A second disadvantage to the prior art digital video server is that video data file retrieval efforts are often duplicated, thereby effectively reducing the video capacity of the network. For example, if two or more workstations wish to simultaneously view the same video data file, then the digital video data file is retrieved twice or more and provided separately to each requesting workstation. Thus, two or more data streams of the same video data file may be simultaneously provided on the network to the requesting workstations. This duplication results in inefficiencies and in taking up valuable network capacity.
A third disadvantage to the prior art video digital server is that each workstation must contain a display adapter for decompressing the video data. These type of adapters are relatively expensive and thus add to the cost of providing full motion video to a number of workstations.